RF transceiver having a T/R switch circuit with high power-handling capability

ABSTRACT

A RF transceiver includes an antenna, a receiver, a baseband circuit, a transmitter, and a T/R switch circuit. The T/R switch circuit is used for coupling the antenna and the receiver or coupling the antenna and the transmitter. The T/R switch circuit is coupled to the baseband circuit so as to receive biases provided from the baseband circuit. The T/R switch circuit includes four transistors and a power detector. The power detector detects the power of the output signal of the T/R switch circuit, so that the baseband circuit can adjust biases provided to the T/R switch circuit according to the power of the output signal.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates a RF transceiver, and more particularly,to a RF transceiver having a T/R switch circuit with high power-handlingcapability.

2. Description of the Prior Art

In the wireless communication system, RF front-end circuit is graduallymaking use of low cost and highly integrated system of complementarymetal oxide semiconductor transistors (CMOS) processed products. Thetransceiver usually uses a common antenna, so the T/R switch circuitfunctions as a transmitting/receiving path for the antenna. Thus, theT/R switch circuit needs enough isolation, low power consumption, andlow insertion loss. In addition, the T/R switch circuit should have highpower-handling capability according to the system requirement.

Filed effect transistor (FET) is generally used for T/R switch circuitbecause its power consumption is almost zero. The GaAs FET is themainstream due to its low insertion loss and high isolation, but onedisadvantage is the negative control voltages. As the development ofCMOS process, many RF sub-circuits are developed, such as low noiseamplifiers, voltage controlled oscillators, and mixers. In view ofsystem on chip (SOC), the T/R switch circuit produced by CMOS process isnecessary. When a transistor functions as a switch, the powerconsumption of the transistor is zero. The state of the transistor isdetermined according to the channel of the transistor. When the channelis turned on, the path of the transistor from the source to the drain isequivalent to a resistor having small input impedance. On the otherhand, when the channel is turned off, the path of the transistor fromthe source to the drain is equivalent to a capacitor having great inputimpedance.

Please refer to FIG. 1. FIG. 1 is a schematic diagram of a conventionalRF transceiver 10. The RF transceiver 10 uses the same antenna 11 toreceive and transmit signals, so the RF transceiver 10 uses the T/Rswitch 12 to switch. The RF transceiver 10 receives a RF signal from theantenna 11. The RF signal is amplified by a low noise amplifier (LNA) 13and is inputted to a mixer 14 so as to generate anintermediate-frequency or baseband signal according to an oscillatingfrequency generated by an oscillator 15. A de-modulator 16 converts theintermediate-frequency or baseband signal to a digital signal andtransmits to a baseband circuit 17. In addition, when the RF transceiver10 sends a signal, the baseband circuit 17 transmits a digital signal toa modulator 18. The modulator 18 converts the digital signal to anintermediate-frequency or baseband signal. A mixer 19 generates a RFsignal according to the intermediate-frequency or baseband signal and anoscillating frequency generated by an oscillator 20. The RF signal isamplified by a power amplifier (PA) 21 and is transmitted by the antenna11.

Please refer to FIG. 2. FIG. 2 is a schematic diagram of a conventionalT/R switch. The T/R switch can be divided into a series type and ashunt/series type. FIG. 2 shows a shunt/series type T/R switch,including four transistors M1, M2, M3, and M4. The transistors M1 and M2are coupled in series for transmission and isolation. When thetransistor M1 is turned on, the transistor M2 is turned off. When thetransistor M1 is turned off, the transistor M2 is turned on. Inaddition, the transistors M3 and M2 are turned on at the same time.Thus, the low impedance is generated at the isolated terminal to providea path to the ground so as to reduce the signal loss. Similarly, thetransistors M4 and M1 are turned on at the same time. For thetransistors M1 and M2, when the channel of the transistor is turned on,the resistance from the drain to the source becomes smaller as the sizeof the transistor increases. Thus, the insertion loss decreases but theparasitic effects of the substrate increases. The insertion lossdecreases to a minimum and increases again because of the parasiticeffects. When the channel of the transistor is turned off, thecapacitance from the drain to the source becomes greater as the size ofthe transistor increases so as to reduce the isolation.

SUMMARY OF THE INVENTION

According to a embodiment of the present invention, a T/R switch circuitwith high power-handling capability, for coupling a first node to asecond node or a third node, comprises a first transistor, a secondtransistor, a third transistor, a fourth transistor, and a powerdetector. A source of the first transistor is coupled to the first node.A drain of the first transistor is coupled to the second node andreceiving a first bias. A gate of the first transistor receives a firstcontrol voltage. A source of the second transistor is coupled to thefirst node. A drain of the second transistor is coupled to the thirdnode and receiving the first bias. A gate of the second transistorreceives a second control voltage. A source of the third transistorreceives a second bias. A drain of the third transistor is coupled tothe second node and receives the first bias. A gate of the thirdtransistor receives the second control voltage. A source of the fourthtransistor receiving the second bias, a drain of the fourth transistoris coupled to the third node and receives the first bias. A gate of thefourth transistor receives the first control voltage. The power detectordetects power of output signals of the second node and the third node soas to adjust the first control voltage, the second control voltage, thefirst bias, and the second bias.

According to another embodiment of the present invention, a RFtransceiver comprises an antenna, a receiver, a baseband circuit, atransmitter, and a T/R switch circuit. The antenna receives or transmitsan analog signal. The receiver converts the analog signal to a digitalsignal. The baseband circuit is coupled to the receiver, for processingthe digital signal. The transmitter is coupled to the baseband circuit,for converting the digital signal to the analog signal. The T/R switchcircuit couples the antenna to the receiver or the transmitter. The T/Rswitch circuit is coupled to the baseband circuit for receiving biasesprovided by the baseband circuit.

These and other objectives of the present invention will no doubt becomeobvious to those of ordinary skill in the art after reading thefollowing detailed description of the preferred embodiment that isillustrated in the various figures and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a conventional RF transceiver.

FIG. 2 is a schematic diagram of a conventional T/R switch.

FIG. 3 is a schematic diagram of a T/R switch 30 according to thepresent invention.

FIG. 4 is a schematic diagram of a RF transceiver according to thepresent invention.

FIG. 5 is a cross-sectional diagram of the transistor M3/M4 in FIG. 3.

FIG. 6 is a table of DC biases for the T/R switch circuit.

FIG. 7 is a schematic diagram of the power transfer curves of the T/Rswitch circuit.

DETAILED DESCRIPTION

Please refer to FIG. 3. FIG. 3 is a schematic diagram of a T/R switch 30according to the present invention. The T/R switch 30 may comprise threenodes N1, N2, and N3. The first node N1 is coupled to an antenna. Thesecond node N2 is coupled to a receiver. The third node N3 is coupled toa transmitter. The T/R switch 30 comprises four transistors M1, M2, M3,M4, and a power detector 31. The source of the first transistor M1 iscoupled to the first node N1. The drain of the first transistor M1 iscoupled to the second node N2 and receives a first bias Vdb. The gate ofthe first transistor M1 receives a first control voltage Vctrl. Thesource of the second transistor M2 is coupled to first node N1. Thedrain of the second transistor M2 is coupled to third node N3 andreceives the first bias Vdb. The gate of the second transistor M2receives a second control voltage Vctrlb. The source of the thirdtransistor M3 receives a second bias Vsb. The drain of the thirdtransistor M3 is coupled to second node N2 and receives the first biasVdb. The gate of the third transistor M3 receives the second controlvoltage Vctrlb. The source of the fourth transistor M4 receives thesecond bias Vsb. The drain of the fourth transistor M4 is coupled tothird node N3 and receives the first bias Vdb. The gate of the fourthtransistor M4 receives the first control voltage Vctrl. In addition, thebias resistors R provide DC bias paths for the transistor M1, M2, M3,and M4.

Key figures of merit of an RF switch are the insertion loss and the 1 dBcompression point (P1 dB). Transistor on-resistance is one of thedominant factors determining insertion loss. Because of this, onlyn-channel MOS transistors are utilized in the design. The drain-to-bodyand source-to-body junction capacitances of the transistors and theassociated parasitic resistances due to the conductive nature of bulksilicon substrates are critical factors determining the insertion loss.For CMOS T/R Switch, maintaining the reverse bias acrosssource/drain-to-substrate junction is a critical factor to limit thepower handling capability due to the conductive nature of siliconsubstrates. Forward biasing the junctions can limit P1 dB as well astrigger latch-up. The switch power handling capability depends on thebias of TX and RX nodes. The power handling capability of the T/R switch30 can be limited by potentially three effects: 1) the input signal isso large that drain/source-to-body junctions of the transistors M1 andM2 are forward biased; 2) the transistors M3 or M4 is unintentionallyturned on and input signal has a path to the ground when the inputsignal is large enough; 3) the voltage across gate oxide is too large toguarantee its long term time-dependent dielectric breakdown (TDDB)reliability. To prevent condition 2 from occurring, the VGS/VGD of thetransistor M3 (DC+AC) should be always kept below the threshold voltageor low enough. Thus, in the embodiment of the present invention, thepower detector 31 can detect the output signal power of the T/R switch30 to adjust the first control voltage Vctrl, the second control voltageVctrlb, the first bias Vdb, and the second bias Vsb, so as to meet therequirement of high transmitted output power and low insertion loss.

Please refer to FIG. 4. FIG. 4 is a schematic diagram of a RFtransceiver 40 according to the present invention. The RF transceiver 40comprises an antenna 32, a receiver 34, a transmitter 36, a basebandcircuit 38 and a T/R switch circuit 30. The T/R switch circuit 30couples the antenna 32 and the receiver 34 or couples the antenna 32 andthe transmitter 36. The output signal information, such as power levelsor linearity, is detected from the output of T/R switch circuit 30 andthus feedbacks into the baseband circuit 38. The power detector 31converts RF signal levels into DC voltage levels. From DC voltagelevels, the baseband circuit 38 can determine the output RF power levelsof the T/R switch circuit 30. According to the requirement oftransmitted output power of the RF transceiver 40, the baseband circuit38 provides the new first control voltage Vctrl, second control voltageVctrlb, first bias Vdb, and second bias Vsb to the T/R switch circuit30. Thus, the T/R switch circuit 30 can exhibit both high power-handlingcapability and low insertion loss.

Please refer to FIG. 5 and FIG. 6. FIG. 5 is a cross-sectional diagramof the transistor M3/M4 in FIG. 3. FIG. 6 is a table of DC biases forthe T/R switch circuit 30. Basically, static biasing can be regarded asa simple representation of adaptive biasing. In order to meet therequirement of the high transmission output power, the higher DC voltagelevels are necessary for the first control voltage Vctrl, the secondcontrol voltage Vctrlb, the first bias Vdb, and the second bias Vsb. Thecharge pump can provide the higher voltages. Under the fixed biascondition, drain/source-to-body junctions of the transistors M1 and M2will not be forward-biased when the voltage swing of the input signal islarge. Furthermore, the transistor M3 or M4 will be unintentionallyturned on. For adaptive biasing, the baseband circuit 38 will receivethe detected DC voltage level from the output of the T/R switch circuit30. Thus, the baseband circuit 38 can determine if the power level ishigh enough. If the T/R switch circuit 30 limits the output power, thebaseband circuit 38 will dynamically adjust all biases of the T/R switchcircuit 30. In this embodiment, the power levels (P1 dB) are 25 dBm, 28dBm, and 30 dBm, for example. The adaptive voltages for the firstcontrol voltage Vctrl, the second control voltage Vctrlb, the first biasVdb, and the second bias Vsb are as shown in FIG. 6.

Please refer to FIG. 7. FIG. 7 is a schematic diagram of the powertransfer curves of the T/R switch circuit 30. Without adaptive biasing,the power transfer curve of the T/R switch circuit 30 is the curve A.With the help of adaptive biasing, the transfer curve of the T/R switchcircuit 30 is the curve B. Thus, the linearity will be greatly improvedand the insertion loss will be reduced in adaptive biasing condition.According to the embodiment of the present invention, the T/R switchcircuit 30 with adaptive biasing has 30.46-dBm input 1 dB compressionpoint and 0.54-dB insertion loss.

In conclusion, the present invention provides a RF transceiver having aT/R switch circuit capable of processing the high power signals. The T/Rswitch circuit uses the adaptive biasing, so the T/R switch circuit hashigh power-handling capability and low insertion loss. The RFtransceiver comprises an antenna, a receiver, a baseband circuit, atransmitter, and a T/R switch circuit. The receiver converts a receivedanalog signal to a digital signal. The transmitter converts the digitalsignal to the analog signal transmitted by the antenna. The T/R switchcircuit is used for coupling the antenna and the receiver or couplingthe antenna and the transmitter. The T/R switch circuit is coupled tothe baseband circuit so as to receive biases provided from the basebandcircuit. The T/R switch circuit comprises four transistors and a powerdetector. The power detector detects the power of the output signal ofthe T/R switch circuit, so that the baseband circuit can adjust biasesprovided to the T/R switch circuit according to the power of the outputsignal. Thus, the T/R switch circuit can achieve the high power-handlingcapability and low insertion loss.

Those skilled in the art will readily observe that numerousmodifications and alterations of the device and method may be made whileretaining the teachings of the invention.

1. A T/R switch circuit for coupling a first node to a second node or athird node, comprising: a first transistor, a source of the firsttransistor being coupled to the first node, a drain of the firsttransistor being coupled to the second node and receiving a first bias,a gate of the first transistor receiving a first control voltage; asecond transistor, a source of the second transistor being coupled tothe first node, a drain of the second transistor being coupled to thethird node and receiving the first bias, a gate of the second transistorreceiving a second control voltage; a third transistor, a source of thethird transistor receiving a second bias, a drain of the thirdtransistor being coupled to the second node and receiving the firstbias, a gate of the third transistor receiving the second controlvoltage; a fourth transistor, a source of the fourth transistorreceiving the second bias, a drain of the fourth transistor beingcoupled to the third node and receiving the first bias, a gate of thefourth transistor receiving the first control voltage; and a powerdetector for detecting power of output signals of the second node andthe third node so as to adjust the first control voltage, the secondcontrol voltage, the first bias, and the second bias; wherein when thepower of the output signals increases, the first bias and the secondbias are increased.
 2. The T/R switch circuit of claim 1, wherein thefirst control voltage and the second control voltage are complementaryvoltages.
 3. The T/R switch circuit of claim 1, wherein the first nodeis coupled to an antenna, the second node is coupled to a transmitter,and the third node is coupled to a receiver.
 4. The T/R switch circuitof claim 1, wherein the power detector transmits the power of the outputsignals to a baseband circuit so that the baseband circuit can adjustthe first control voltage, the second control voltage, the first bias,and the second bias.
 5. A RF transceiver, comprising: an antenna forreceiving or transmitting an analog signal; a receiver for convertingthe analog signal to a digital signal; a baseband circuit, being coupledto the receiver, for processing the digital signal; a transmitter, beingcoupled to the baseband circuit, for converting the digital signal tothe analog signal; and a T/R switch circuit, comprising: a firsttransistor, a source of the first transistor being coupled to theantenna, a drain of the first transistor being coupled to the receiverand receiving a first bias provided by the baseband circuit, a gate ofthe first transistor receiving a first control voltage; a secondtransistor, a source of the second transistor being coupled to theantenna, a drain of the second transistor being coupled to thetransmitter and receiving the first bias, a gate of the secondtransistor receiving a second control voltage; a third transistor, asource of the third transistor receiving a second bias provided by thebaseband circuit, a drain of the third transistor being coupled to thereceiver and receiving the first bias, a gate of the third transistorreceiving the second control voltage; a fourth transistor, a source ofthe fourth transistor receiving the second bias, a drain of the fourthtransistor being coupled to the transmitter and receiving the firstbias, a gate of the fourth transistor receiving the first controlvoltage; and a power detector for detecting power of output signals ofthe receiver and the transmitter so as to adjust the first controlvoltage, the second control voltage, the first bias, and the secondbias; wherein when the power of the analog signal increases, thebaseband circuit increases the first bias and the second bias.
 6. The RFtransceiver of claim 5, wherein the baseband circuit adjusts the firstcontrol voltage, the second control voltage, the first bias and thesecond bias according to the power of the analog signal.
 7. The RFtransceiver of claim 5, wherein the first control voltage and the secondcontrol voltage are complementary voltages.
 8. A T/R switch circuit forcoupling a first node to a second node or a third node, comprising: afirst transistor, a source of the first transistor being coupled to thefirst node, a drain of the first transistor being coupled to the secondnode and receiving a first bias, a gate of the first transistorreceiving a first control voltage; a second transistor, a source of thesecond transistor being coupled to the first node, a drain of the secondtransistor being coupled to the third node and receiving the first bias,a gate of the second transistor receiving a second control voltage; athird transistor, a source of the third transistor receiving a secondbias, a drain of the third transistor being coupled to the second nodeand receiving the first bias, a gate of the third transistor receivingthe second control voltage; a fourth transistor, a source of the fourthtransistor receiving the second bias, a drain of the fourth transistorbeing coupled to the third node and receiving the first bias, a gate ofthe fourth transistor receiving the first control voltage; and a powerdetector for detecting power of output signals of the second node andthe third node so as to adjust the first control voltage, the secondcontrol voltage, the first bias, and the second bias; wherein the powerdetector transmits the power of the output signals to a baseband circuitso that the baseband circuit can adjust the first control voltage, thesecond control voltage, the first bias, and the second bias.
 9. The T/Rswitch circuit of claim 8, wherein the first control voltage and thesecond control voltage are complementary voltages.
 10. The T/R switchcircuit of claim 8, wherein the first node is coupled to an antenna, thesecond node is coupled to a transmitter, and the third node is coupledto a receiver.